Twilkes
Guru
FFS this bike is falling apart..... 
There is some play in the crank - the chainrings, cranks and pedals all move together, maybe 0.5-1mm movement at the edge of the chainring when I try to pull the pedals back and forth sideways.
Noticed some squeaking about a week ago, and the last few days have noticed a looseness while stationary with my feet on the pedals, which led me to check whether everything was tight or not.
I'm thinking bottom bracket, although I don't really know what that is - could it just need tightening or is it likely to need replacing?
If replacement, what would I need to order? It's a 2007 Raleigh Pioneer that had Shimano hardware if that helps with identifying likely bottom brackets (whatever they are).

There is some play in the crank - the chainrings, cranks and pedals all move together, maybe 0.5-1mm movement at the edge of the chainring when I try to pull the pedals back and forth sideways.
Noticed some squeaking about a week ago, and the last few days have noticed a looseness while stationary with my feet on the pedals, which led me to check whether everything was tight or not.
I'm thinking bottom bracket, although I don't really know what that is - could it just need tightening or is it likely to need replacing?
If replacement, what would I need to order? It's a 2007 Raleigh Pioneer that had Shimano hardware if that helps with identifying likely bottom brackets (whatever they are).